Some image forming apparatuses include a contact charger which makes a charging member contact with a substrate such as an image bearing member to charge the image bearing member. In most contact chargers, an AC voltage is applied upon the charging member to charge the image bearing member, which allows the portions of the image bearing member to be charged more uniformly, compared with the case in which a DC voltage is applied upon the charge member.
Disadvantageously, the application of the AC voltage upon the charging member induces undesirable noise, or charge noise. The charge noise is a vibroacoustic signal of the charging member and/or the image bearing member, which is caused by an attractive force generated between the charging member and the image bearing member and periodically changing in synchronism with the frequency of the AC voltage.
JP 05-210281 A discloses an image forming apparatus capable of preventing or reducing the generation of the charge noise. The charging member of this apparatus includes a metal core or support member, an elastic layer supported on the metal core, and a charging layer disposed around the elastic layer for the contact with the image carrier. According to this apparatus, the generation of the charge noise is minimized by an appropriate design in thickness of the charging layer.
Practically, however, it is very difficult to provide the charging layer of the charging member with a function for reducing the generation of the charge noise. Specifically, the charging layer of the charging member needs to have characteristics of uniformly and stably charging the image bearing member, preventing a permanent compressive deformation due to the constant abutment with the image bearing member, and preventing a development of an electric leakage between the image carrier and the charging member. In addition to those indispensable features, it is very difficult in design and/or manufacture to provide the charging layer with another function of preventing the charge noise.